Abstract: The present invention is a method for producing a resin-molded body for a hollow structure having a cavity and an opening connecting the cavity and the outside. The method includes the steps of: creating a hollow core corresponding to the shape of the cavity; creating an outer mold corresponding to the external shape of the hollow structure; setting the core in the outer mold and injecting a fluid of a soft-resin material into the space between the outer mold and the core; demolding a soft-resin-molded body together with the core from the outer mold after the soft-resin-molded body is formed by curing the soft-resin material, applying an external force to the soft-resin-molded body to deform the soft-resin-molded body so as to break the core into fragments; and discharging the fragments of the core through the opening of the soft-resin-molded body to the outside.